In this work we conclude the analysis of our CO line survey of Luminous Infrared Galaxies (LIRGs: L_{ IR}>=10^{ 11}L_{ sol}) in the local Universe (Paper\,I), by focusing on the influence of their average ISM properties on the total molecular gas mass estimates via the so-called X_{ co}=M(H_2)/L_{ co,1-0} factor. One-phase radiative transfer models of the global CO Spectral Line Energy Distributions (SLEDs) yield an X_{ co} distribution with: <X_{ co}>\sim(0.6+/-0.2) M_{ sol}(K km s^{ -1} pc^2)^{ -1} over a significant range of average gas densities, temperatures and dynamical states. The latter emerges as the most important parameter in determining X_{ co}, with unbound states yielding low values and self-gravitating states the highest ones. Nevertheless in many (U)LIRGs where available higher-J CO lines (J=3--2, 4--3, and/or J=6--5) or HCN line data from the literature allow a separate assessment of the gas mass at high densities (>=10^{ 4} cm^{ -3}) rather than a simple one-phase analysis we find that { \it near-Galactic X_{ co} (3-6)\, M_sol\,(K\,km^{ -1}\,pc^2)^{ -1} values become possible.} We further show that in the highly turbulent molecular gas in ULIRGs a high-density component will be common and can be massive enough for its high X_{ co} to dominate the average value for the entire galaxy. ......... ...this may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.
Categories: astro-ph.CO astro-ph.GA